The present invention relates to (1) a phototransistor which can be used as a single photosensor having both switching and amplifying functions as well as an integrated one-dimensional or two-dimensional sensor, (2) a photosensor array using phototransistors, especially a one-dimensional contact photosensor array which is configured so that a lensless sensor, which is lower in price and can be used for facsimile terminal equipment and image scanners, can be produced, and (3) a two-dimensional photosensor which is suited to the image input unit or data reader of a personal computer, word processor, or work station.
Conventional phototransistors, as indicated on page 779 of the VLSI General Dictionary (published by "Science Forum " Mar. 31, 1988, Junichi Nishizawa), basically have the structure of bipolar transistors. Each phototransistor mentioned above has a structure that light is irradiated on the base, and minority carriers are introduced to the base by light.
Japanese Patent Laid-Open 1984-110179 indicates that, for forming a semiconductor device such as a solar cell or photodiode, an "a-Si/metal interface reaction layer" ("a-" means "amorphous") is used as a transparent electrode. Phototransistors formed by the conventional technology, which are basically bipolar transistors, have problems such that (1) it is difficult to improve the light efficiency and directivity, (2) the input impedance is low, and (3) each phototransistor cannot be expanded to a field effect transistor.
A conventional one-dimensional contact photosensor is described in JARECT Vol. 16, 1984, pages 290 to 299. The photosensitive unit employs amorphous silicon (a-Si) photodiodes, which are arranged to form independent islands. An external drive circuit is provided to fetch a photo signal from each diode by scanning the diodes sequentially. An integrated circuit is generally used as an external drive circuit. In addition to the photodiodes mentioned above, thin film transistors using amorphous silicon (a-Si) are often used to provide a switching function.
Another example of a one-dimensional photosensor formed by the conventional technology is a photosensor called a complete contact sensor which requires no lens. This is described, for example, on pages 53 to 56 of the collection of manuscripts for the symposium "How are amorphous silicon devices progressed?" sponsored by Electrophotography Institute (Osaka Scientific Technology Center, May 24, 1985). In this example, the photosensitive unit is comprised of photodiodes. Each diode is provided with a hole at its center to let light pass through it. Light passing through the hole is reflected off of a manuscript and irradiated to the diode unit to be converted to an electric signal.
However, the photosensitive unit of a one-dimensional photosensor by the conventional technology, which uses photodiodes, requires an external drive circuit to be independently provided because of the lack of the switching function, resulting in problems of a complicated configuration and higher price.
The two-dimensional image input unit of a conventional personal computer, word processor, or work station is an input unit using the method of scanning a manuscript by mechanically scanning the one-dimensional image sensor.
The two-dimensional image input unit formed by the conventional technology mentioned above uses both mechanical scanning and electrical scanning, causing limits to miniaturization of the unit, speed up of the reading rate, and color display. For example, as to the reading speed, it takes about 15 sec per sheet of A4 type (about 210 mm by 297 mm) specified by JIS at a resolution of 8 lines/mm. Color display requires mechanical scanning to be repeated three times, and the reading time is increased to more than 3 times and the freedom degree for filter configuration is lost.